Audio and video equipment is being developed which is capable of bidirectional transmission of information via networks using an IEEE 1394 serial data bus. When transmitting data over such a bus, an isochronous transfer mode, used for realtime transmission of comparatively large quantities of video data, audio data and similar, and an asynchronous transfer mode, used for reliable transmission of still images, text data, control commands and similar, are provided, and dedicated frequency bands are used for each of these modes in transmission.
FIG. 1 is a diagram showing an example of connections using such an IEEE 1394 bus; source equipment a, which is equipment sending data, input equipment (sink equipment) b, which is equipment receiving data sent from the source equipment a, and a controller c which controls data transmission between the equipment a and b, are connected by an IEEE 1394 bus d. At this time, when for example audio data is to be transmitted between the equipment a and b under control of the controller c, the controller c secures an isochronous transfer channel on the bus d, and after establishing a connection between the equipment a and b to enable transmission over this channel, causes transmission from the source equipment a to the input equipment b to be initiated. The source equipment a or the input equipment b may also serve as the controller.
When transmitting data between the source equipment a and input equipment b in this way, a transmission method, in which control commands in what is called an “AV/C command transaction set” are used, is applied to, for example, audio and video equipment. The details of the AV/C command set are described in the “AV/C Digital Interface Command Set General Specification,” published by the 1394 Trade Association.
When transmitting audio data or other streaming data from source equipment to input equipment, processing must be performed by the input equipment according to the type of streaming data transmitted. Hence when there are changes in the format or other parameters of the streaming data during transmission, this must be detected by the input data, and switching processing must be performed to change the processing state.
FIG. 2 is a diagram showing one example in which the data format changes during transmission of streaming data via a bus line. Suppose for example that the source equipment a is a disc reproduction apparatus, and that audio data (streaming data) comprising content A and content B is recorded on the disc from which data is being reproduced by the disc reproduction apparatus. Suppose that content A and content B are content recorded as audio data for output as streaming data in different formats. Further, suppose that the input equipment b processes the input audio data for output, and is provided with the functions of an amplifier to output the data to a connected speaker or similar. In such a configuration, streaming data reproduced by the source equipment a from the disc is transmitted to the input equipment b via the bus line.
Suppose that in such transmission, reproduction of content A ends at time ta, and processing proceeds to reproduction of content B. At this time, the input equipment b must change the settings of internal circuitry and similar from the processing state for processing of content A to the processing state for processing of content B, and so the audio of content B is output from the speaker at the time tb at which these preparations are completed.
Hence during the interval from time ta, at which the format changes, until time tb, the audio of content B is not output, and so the problem of loss of the beginning of the content occurs.
Such problems are particularly prominent when what is called rate control (flow control) processing is executed between the source equipment and the input equipment. FIG. 3 is a diagram showing a state in which rate control is performed. This rate control is flow control processing to adjust the rate at which streaming data is output from the source equipment a, in keeping with the rate of processing of the streaming data by the input equipment b. In this example, the input equipment b comprises a buffer memory m which temporarily accumulates transmitted streaming data; the input equipment b sends rate control data to the source equipment a such that the amount of data accumulated in this memory m is substantially constant, and the source equipment a controls the output rate based on this rate control data when sending streaming data to the input equipment b.
In this case, when the format of the input data changes, if the buffer memory m does not output all the data accumulated in the memory to temporarily empty the buffer, data in another format cannot be accumulated. For this reason, when there is a change in format from content A to content B as shown in the above-mentioned FIG. 2, data accumulated in the memory m at time ta at which the format of the transmitted data changes must all be output before the content B can be input. Hence when control such as rate control is being executed, there is the problem that a comparatively long time is necessary before the changed format can be accommodated (that is, the time interval from time ta to time tb in FIG. 3).
The explanation thus far has described problems for the case of transmission of streaming data over a network connected by an IEEE 1394 bus line; but when transmitting streaming data over other networks, similar problems occur if the format of the streaming data changes.